Heretofore, heat sealable tea bag papers have comprised both single phase and multi-phase sheet material. Both materials have included non-heat seal fibers such as cellulosic fibers in combination with heat seal fibers. The particular heat seal fibers used have included thermoplastic fibers, such as the fibers of a copolymer of polyvinyl acetate, commonly referred to as "vinyon," and polyolefin fibers such as fibers of polyethylene and polypropylene. These synthetic heat seal fibers are typically smooth rod-like fibrous materials exhibiting a low specific surface area. They form a highly porous and open structural arrangement which, despite their hydrophobic character, permit adequate liquid permeability and transmission of both hot water and tea liquor through the sheet material during the normal brewing process. During manufacture the sheet material is dried by a conventional heat treatment resulting in a slight contraction of the heat seal thermoplastic fibers that maintains and enhances the desired open distribution of the heat seal particles throughout the sealing phase of the web.
In recent years, fibrillar materials formed from polyolefins and similar polymers have been introduced in the paper industry. These materials, commonly referred to as "synthetic pulps", exhibit certain processing advantages over the smooth rod-like synthetic fibers used heretofore. The synthetic pulps exhibit a fibrilliform morphology and resultant higher specific surface area. Additionally, they are more readily dispersible in water without the need for additional surface active agents and, although hydrophobic in nature, they do not dewater as rapidly as conventional synthetic fibers and therefore avoid plugging problems in lines, pumps, etc., within the paper-making machine. Further, these synthetic particles do not exhibit the tendency to "float out" in chests and holding tanks used in the typical paper-making process. For these reasons the synthetic pulps exhibit a potential for use as the heat seal component of infusion package materials, particularly since they provide substantially improved wet seal strength under end use conditions, that is, improved wet seal strength in a hot aqueous liquid environment and improved resistance to seal delamination under boiling and steaming test conditions.
Despite the apparent advantages evident in the use of synthetic pulp for heat seal infusion paper application, it has been found that such material exhibits a significant disadvantage with respect to its infusion properties and its wettability. This disadvantage relates directly to its usefulness in the paper-making process, that it, its fibrilliform structure and high specific surface area. When the synthetic pulp is heat treated, as in the conventional drying operation, it tends to soften and flow, typically forming a film, albeit discontinuous, particularly in the heat seal phase of a multi-phase sheet material. Unlike the highly porous and open web structure formed by the larger and smoother synthetic fibers, the high surface area pulp with its lower density, smaller particle size and more numerous particles results in a closed, low permeability structure. In addition, the hydrophobic nature of the basic polymer inhibits water permeability and any surfactant added to the synthetic pulp is neutralized during the drying process. The result is that certain areas of the web surface are rendered water impermeable substantially retarding or inhibiting infusion and reducing the water permeability and wettability of the material. In use, the non-wetted or partially wetted areas of the web material are easily observed as opaque areas on the sheet while the thoroughly wetted areas exhibit a transparent appearance. The reduced wettability of the web material coupled with its mottled opaque appearance influences the aesthetic attractiveness of the product under end use conditions and, therefore, its acceptability by the consumer.